1. Field of the Invention
This invention relates to an image reading system for reading an image by scanning a light-transmission original such as a photographic film and more particularly to the automatic focusing arrangement of the image reading system.
2. Description of the Related Art
The known systems of the above-stated kind for reading a light-transmission original such as a 35-mm photographic film include, for example, a drum scanner which is arranged to wind the film on a rotary drum, to rotate the drum and to scan the original by moving the photo-electric conversion part thereof along the drum. In this case, an automatic focusing action is not particularly required.
However, in cases where the light-transmission original, such as the 35-mm photographic film, is to be read at a high degree of resolution, the conventional system has presented the following problems:
In the case of a reversal film (positive film), each frame is in most cases set on a mount. However, the thickness of the mount varies. If the film is handled in a state of being mounted on a mount, the position of the film surface fluctuates in the direction of the optical axis within a range of several mm. Therefore, focus adjustment is necessary for reading out a sharp image.
Further, the film tends to warp irrespective as to whether or not it is mounted on a mount. It is, therefore, difficult to keep the film surface position unvarying. Further, the film position has been greatly affected by the play or uneven degree of precision of the mechanical part of the system.
To solve this problem, it has been practiced in general to stick the film to a glass surface or have the film sandwiched in between two sheets of glass for accurate positioning of the film surface and for preventing it from warping. It is, however, troublesome to take out the film from the mount and to stick the film to the glass. Sandwiching the film in between two sheets of glass brings about Newton rings or tends to have dust sticking either to the glass surface or to the film.
Further, manual focus adjustment for every mount is troublesome, and it not only takes time but also is difficult to accurately perform.
It has been another problem that the focus position for each part of the film surface varies due to the various aberrations of the image forming lens such as curvature of field, astigmatism, chromatic aberration, etc. For example, the focal positions of the color separation images of three colors red (R), green (G) and blue (B) deviate from each other.
Further, while some films have object images formed in a sharply in-focus state including high-frequency components in a large amount, others are not. Automatic focus adjustment cannot be accurately carried out, therefore, if the lens is focused on a film having an image not sharply formed, because, in such a case, it is hardly possible to obtain a high degree of sharpness.